Off-axis three-mirror imaging system is a kind of off-axis reflective system. If freeform surfaces are used in off-axis three-mirror imaging systems, the aberrations of the system can be significantly reduced, while the system specifications can be greatly improved.
In conventional off-axis three-mirror optical system with freeform surfaces, the three mirrors are separated in space and they have different freeform surface analytical expressions. If the primary and tertiary mirrors share a same freeform surface expression and are fabricated on a single substrate, the difficulty of system alignment and fabrication as well as the cost for the testing of the system can be reduced.
However, since the primary mirror and the tertiary mirror are located generally far away from each other on a single element in this kind of system, a size of the conventional off-axis three-mirror optical system with freeform surfaces is large, and a structure of the conventional off-axis three-mirror optical system with freeform surfaces is not compact. In addition, a volume of the single element is large, therefore the surface sag at the edge of the surface is large, which increases the difficulty for fabrication and testing.